To refinish or repair a finish on vehicles, such as a basecoat/clearcoat finish on automobile or truck bodies, different fast-drying coating compositions have been developed. A number of pigmented and clear air-dry acrylic urethane coatings have been used in the past to repair basecoat/clearcoat finishes, but none meet the rapid drying times and early film hardness that are desired, while also meeting today's performance requirements, such as ease of application, moisture resistance, intercoat adhesion, durability, weathering resistance, and outstanding appearance of the overall finish, such as gloss and DOI (distinctness of image).
A concern to a refinish customer, which is typically the vehicle owner, is that the coating in use has excellent durability and weatherability and an attractive aesthetic appearance.
A key concern of the automobile and truck refinish industry is productivity, i.e., the ability to complete an entire refinish operation in the least amount of time. To accomplish a high level of productivity, any coatings applied need to have the ability to dry at ambient or elevated temperature conditions in a relatively short period of time. The term “dry” means that the resulting finish is physically dry to the touch in a relatively short period of time to minimize dust or dirt pick-up, and, in the case of the basecoat, to allow for the application of the subsequent clear coat.
Before any further work can be done to the finish, the finish must also be sufficiently hard to sand or buff to improve the gloss or to remove minor imperfections. Conventional finishes, however, are unable to cure to a sufficiently hard state in a relatively short period of time, and thus the productivity of a refinish operation is still lacking, since the vehicles cannot be worked on quickly after application.
One approach that has been used to improve the initial film hardness (i.e., film Tg) of a clearcoat composition involves replacing a portion of the conventional polyisocyanate crosslinking agent such as hexamethylene diisocyanate (HDI) trimer with a relatively hard or rigid material such as isophorone diisocyanate (IPDI) trimer. Unfortunately, IPDI has a much slower curing rate than that of HDI. Consequently, much of the IPDI does not get crosslinked and incorporated into the crosslinked coating film, and the Tg of the coating is not significantly raised.
U.S. Pat. No. 6,472,943, issued to Huynh-Ba on Oct. 29, 2002, proposes an improved method for incorporating isophorone polyisocyanates into a refinish coating by significantly increasing the level of catalyst in the coating system. Although the above-cited composition provides good initial film hardness, better initial film hardness is still desired. Accordingly, a continuing need still exists for a coating composition suited for use as a clearcoat in automotive refinishing that offers very fast dry-to-touch times and higher film hardness after application when cured at ambient or slightly elevated temperatures, so that a vehicle can be moved quickly out of the paint booth so another vehicle can be painted and also can be worked on quickly after application.
The novel coating composition of this invention has the aforementioned desirable characteristics.